1. Technical Field
The present invention relates to a ballast used to drive high intensity discharge lamps and, more particularly to the same HID ballast being used for a bank of interconnected light emitting diodes (LEDs).
2. Description of Related Art
A high-intensity discharge (HID) lamp produces light by means of an electric arc between tungsten electrodes housed inside a fused quartz or fused alumina arc tube. The tube is filled with both gas and metal salts. The gas facilitates an initial strike or ignition of the arc. Once the arc is started, the arc heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. In typically 1 to 2 minutes, a low powered 70 W HID lamp warms up to produce its rated light output. When the HID lamp is initially cool, an ignition voltage of 4000 volts for instance is typically required to ignite the HID lamp. A re-ignition for the same lamp when the lamp is still hot, may require up to 20,000 volts for re-ignition to occur. The re-ignition when the lamp is still hot may also require a different frequency or phase characteristic for the ignition voltage to avoid risk of blowing up the HID lamp. Ballasts and lamps with hot re-strike capability are much more expensive then ballasts and lamps without hot re-strike capability.
After ignition, the HID ballast provides alternating current to the lamp at low voltage, e.g. 20-100 Volts. The physical properties of the HID lamp typically determine the operating voltage across the HID lamp.
There are two types of HID ballasts, generally termed “low” and “high” frequency ballasts. The “low frequency ballast” includes a rectifier circuit which rectifies the alternating current of the power line to direct current. The direct current is input to a circuit that performs “power factor correction” (PFC). “Power factor” is a figure of merit indicating to what extent the current and the voltage are in phase. The PFC circuit is followed by a “buck converter” providing a current source and performing a DC-DC step down conversion. The “buck converter” is followed by a full-wave bridge operating as an “inverter” outputting a low frequency, e.g. 160 Hz. square wave as input to the discharge lamp.
The “high frequency ballast” includes a rectifier circuit followed by a PFC circuit followed by either a “half bridge” or a “full bridge” circuit operated at high frequency, 100 kHz. or greater. The ignition method used in high frequency ballasts may include resonant ignition, using a high frequency sine wave or semi-resonant ignition using pulses superimposed on the peaks of a high frequency sine wave.
Modern HID ballasts are microprocessor controlled, ie. circuit blocks include transistor switches, e.g. gates of MOSFETS, which are controlled by a microprocessor.
HID lamps are widely used for illumination in public areas because of the high efficiency available, e.g 100-140 lumens/watt. However, under a drop of mains voltage, when hot re-strike is not used or unavailable, HID lamps remain off for five to ten minutes while they cool down before re-ignition. While HID lamps are in the process of cooling down, other lighting must be used which supplies sufficient light just after the mains voltage comes back on. Quartz-halogen lamps are often used for emergency lighting which are lit while the HID lamps are cooling down and waiting for re-ignition. The quartz-halogen lamps require different wiring and fixtures from the HID lamps.
Thus there is a need for and it would be advantageous to have a system and method for providing emergency lighting during the time period after a drop in mains voltage and before re-ignition of the HID lamps without requiring use of different circuitry, additional infrastructure or hot re-strike capability.
The ballast used to ignite and operate an HID lamp is very different from and should not be confused with the ballast and starter used to operate a fluorescent lamp. A fluorescent lamp uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that causes a phosphor to fluoresce, producing visible light. The mercury atoms in the fluorescent tube must be ionized before an arc can “strike” within the tube. A combination filament/cathode at each end of the lamp in conjunction with a mechanical or automatic switch initially connects the filaments in series with the ballast and thereby preheat the filaments prior to striking the arc. The preheating typically takes between 2 to 3 seconds which is followed by striking of the warmed mercury vapor inside the fluorescent lamp. The strike is performed after preheating the lamp to avoid damage to the fluorescent lamp. The strike is typically performed by using another controlled circuit portion of the fluorescent ballast circuit known as a starter. The peak voltage of the pulse provided by the starter is used to strike the warmed mercury vapor inside the fluorescent lamp and is typically 1200 to 1500 volts. Light produced by the fluorescent lamp after application of the starter circuit is virtually instantaneous. A typical 40 W 48″ fluorescent tube, starts at 400-650 Volts and has about a 93V working voltage. High frequency ballasts for fluorescent lamps run at 20-60 kHz. Fluorescent lamps immediately re-ignite if turned off.